1. Field of the Invention
The invention disclosed in this specification relates to element isolation of a semiconductor element used for a semiconductor device. In addition, the invention relates to a semiconductor element formed by element isolation.
2. Description of the Related Art
A thin film transistor, which is one kind of semiconductor element, is generally referred to as a TFT and is widely known as a switching element used for an active matrix display device. Conventionally, in order to perform element isolation during a manufacturing process of a semiconductor element such as a thin film transistor, the following methods have been employed: a method of isolating a semiconductor film into an island-shaped pattern by patterning and etching (hereinafter, referred to as a photolithography method), a method of directly forming an island-shaped pattern by a droplet discharging method, a method of selectively forming an oxide film for element isolation by thermal oxidation, which is referred to as LOCOS (local oxidation of silicon) (hereinafter, referred to as a LOCOS method), or the like.
However, in a case of a droplet discharging method, there is a problem in that miniaturization of an element is difficult in comparison with a LOCOS method. In a case of a photolithography method, there is a problem that, when an island-shaped pattern having a side surface which is almost perpendicular (90°±10°) to a surface of a base film formed over a substrate is formed, not only a semiconductor film but also the base film is etched, and accordingly, a corner portion (edge portion) of the formed island-shaped pattern becomes a peak shape as shown in FIG. 5A of Patent Document 1 (Japanese Published Patent Application No. H7-094756).
Even if an island-shaped pattern can be formed without formation of a peak shape, the base film is etched by cleaning the formed island-shaped pattern with hydrofluoric acid. Accordingly, a corner portion of the island-shaped pattern becomes a peak shape. Cleaning with hydrofluoric acid is a cleaning method which is often performed during a manufacturing process of a semiconductor element in order to remove a natural oxide film formed over a surface of silicon and in order to obtain a clean surface.
When a gate insulating film is formed covering the island-shaped pattern by a CVD method after the peak shape is formed as described above, there are problems in that a thickness of the formed gate insulating film is not uniform and a thin portion is generated, a crack is generated, and the like, as shown in FIG. 5B of Patent Document 1. It is considered that the problems become noticeable when an attempt to make the thickness of the gate insulating film thin and uniform is made and cause leak current through the gate insulating film. In the invention described in Patent Document 1, in order to solve the problems, after a peak shape is formed, a thin oxide film is formed over a surface of an island-shaped pattern by emitting infrared light or a laser in an oxygen atmosphere or by heating in an oxygen atmosphere, and furthermore, an insulating film is formed by a CVD method or the like. In this case, however, because the problem of the formation of the peak shape cannot be solved, there are problems in that forming the gate insulating film thinly and uniformly is difficult and that the number of steps for forming the gate insulating film is increased.
In order to perform element isolation without formation of an island-shaped pattern by a LOCOS method, it is necessary to oxidize silicon at a high temperature of approximately 1000° C. in an oxygen atmosphere. Therefore, a substrate which cannot withstand a high temperature of 1000° C., such as a plastic substrate or a glass substrate typified by a nonalkali glass substrate, cannot be employed as a substrate to be used. Although a quartz substrate (also referred to as a silica glass substrate) which can withstand a temperature of 1000° C. is sold, such a quartz substrate is more expensive than the glass substrate.
In recent years, a method of forming a gate insulating film of a semiconductor element by using a high-density plasma treatment apparatus has attracted attention. For example, in Patent Document 2 (Japanese Published Patent Application No. 2004-319952), forming a silicon nitride layer which becomes a gate insulating film over a semiconductor layer by directly reacting nitrogen activated by plasma excitation with the semiconductor layer of an SOI (silicon on insulator) substrate is disclosed. In addition, in Patent Document 2, the semiconductor layer of the SOI substrate is isolated in each element formation region. However, as an isolation method, only formation of a trench in the semiconductor layer by a dry etching method or formation of an element isolation region by an STI (shallow trench isolation) method or the like is described.